As an example of a compact high-resolution SEM, reference is made to FIG. 1 in JP-B No. 1681/1995. This compact high-resolution SEM is characterized by the fact that an electrostatic lens is provided for focusing electrons into an electron beam, where an electrode outside an objective lens is 0 V and a positive high voltage is applied inside the objective lens to accelerate electrons. However, this method has a problem in that, since the column has no vacuum pump and a secondary electron detector is located in a place different from the column, the apparatus size must be relatively large. Besides, this configuration makes it difficult to provide a number of columns and to move a column for purposes of extending the working distance of the measurement.
On the other hand, in an optical column of a charged particle beam apparatus as disclosed in JP-T No.522381/2003, the portion from an electron source to an objective lens inside the column is housed in a beam booster to create a high acceleration condition inside the column, in which detection is carried out using a collector containing fluorescent material for secondary electrons from a sample and a light guide to the outside. However, in accordance with this method, since the electron source is housed in the booster, a phase difference occurs between the beam booster and accelerator, and electrons emitted from the electron source are immediately accelerated by the beam booster electric potential. Consequently a strong electron lens effect is produced in the vicinity of the electron source. This results in a significant aberration which makes it difficult to achieve high resolution. For a compact apparatus, the beam booster must be small, and, thus, the beam booster's withstand voltage cannot be high. Therefore, a high voltage cannot be applied to the beam booster, and an electron beam emitted from the electron source cannot be accelerated sufficiently, so that it is impossible to achieve high resolution. Also, since the electron source is covered, it is difficult to create an ultra-high vacuum atmosphere necessary for operation of a high performance electron source.
A widely known approach to measuring the 3D structure of a sample through use of a SEM is that a cosine distribution, in which an emission angle distribution of secondary electrons is almost proportional to Cos θ with respect to the sample surface normal line, is utilized to measure a deviation in secondary electron emission distribution and to calculate a sample surface inclination to estimate a 3D structure. Usually, the overall size of this type of apparatus is large, because more than one secondary electron detector is provided. SEM systems which perform this measurement through a compact column incorporating detectors are shown in U.S. Pat. No. 5,644,132 and JP-A No.141015/2002. In these systems, a magnetic objective lens is used as a main focusing means for an electron beam, and secondary electrons from the surface of a sample irradiated with this beam are accelerated by an electric field generated at the bottom of the objective lens. The secondary electrons are passed through the objective lens, and the intensity distribution in each of the four directions is measured by a quadrant detector located above the lens to calculate secondary electron emission angle deviations. However, the magnetic lens is used mainly for focusing electrons into an electron beam, and the coil and magnetic circuit cannot be small in size. This makes it difficult to make the apparatus more compact. In addition, this method has a problem in that, while secondary electrons are passing through the magnetic field of the objective lens, their orbit rotates, so that this rotation angle must be taken into consideration in the calculation of a 3D structure. Therefore, there is a secondary electron energy distribution caused by each change in lens intensity, and the rotation angle varies with energy, resulting in a deterioration in measurement accuracy. Besides, since there is no electrode for controlling the charge caused by an electron beam, it is difficult to measure a semiconductor sample containing an insulating material.